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Full text of "Handbook Of Chemical Engineering - I"

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CONCENTRATION                                      337
The chief commercial application of the method is in separating blende with little or no occluded pyrite, from the latter. The process avoids the roast necessary fo-i separating these substances magnetically. The process cannot be applied to material coarser than six mesh when the capacity of the standard Huff machine of about 10ft. height and about 5 ft. width may be as high as 35 tons per day of 24 hr. The rougher portion of the separator consists of six separator bars which splits the entering stream into a conductor and non-conductor portion. The finisher part of the unit consist of two parts of six bars each, one set being for conductor material and the other for the non-conductor material. The middlings made by each side are returned to the top of the finisher for retreatment. The machine will not treat the finest dust and on material ranging from 80 to 200 mesh the capacity under favorable conditions may be as high as 4 tons a day.
Flotation.—Flotation takes advantage of the affinity of sulphide mineral and other substances for oils, greases and gases or of the repulsive effect of the same substances on the surface tension "skin" of a water surface. Modern applications of the principles employed in flotation fall under three heads. (1) The direct floating of powdered dry metallic substances on the surface of water. Some success has attended the use of this method with graphite and molybdenite. (2) Recovery of valuable material by a grease-coated surface. This method has been successfully used for recovering diamonds in the South African field. Haul-tain and Stovel employ a greased endless belt. At a trial of their invention at the Mammoth mill in the Coeur d'Alene region, Idaho, the endless belt with a cotton center and wool edges was placed in a highly inclined position and fed by a down-pouring stream of slime the belt moving upwards against the flow. Mineral coated grease was taken off at the top of the belt and removed to a steam heated tank filled with water where the grease was melted and the concentrate settled to a removing worm at the bottom. The grease after being floated off the water was congealed and again applied at the bottom of the belt. (3) The methods most commonly employed consist in making an air-oil-sulphide froth which rises to the top of the water by its superior buoyancy where it can be skimmed off or caused to flow to any desired collection point.
In the Callow process air is introduced from a compression source through a porous diaphragm in the bottom of the series of tanks or compartments employed. The air bubbles in their ascension become oiled by the oil added to the stream of slime flowing into the apparatus and by the superior affinities of the air, oil and sulphides for one another form a "froth" so that a substance like beaten white of egg rises to the surface of the separating compartments through its superior buoyancy The oil and sulphides form the "framework" of the froth Some gangue is also carried up with the froth but the amount of this compared to what goes to waste from the bottom of the apparatus is small. In the Ruth apparatus there are separate aeration chambers and the aeration means consist of a hollow vertical shaft driven by belting or other means. Air is pulled down through the hollow shaft by means of the centrifugal force created by a disc secured to the bottom of the shaft and with ports at the periphery connected with the central opening in the shaft. The slimes and oil are introduced into the aerating compartment and pass from this into a compartment facing it where the froth rises and discharges.
In the chief class of flotation machines the air is actually beaten into the oiled froth
by revolving beater or impellers.    Beaters mounted on horizontal shafts have received
much attention from inventors in recent years.    They revolve at lower speeds than
the machines using vertical impellers and are arranged to trap the air by having the
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